JP2010102096A - Zoom lens and image capturing apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a zoom lens, which is compact, holds satisfactory optical performance, and also reduces cost. <P>SOLUTION: The zoom lens includes, in order from an object side, a first positive lens group G1, a second negative lens group G2 which moves to perform variable power, a diaphragm, a third positive lens group G3, and a fourth positive lens group G4 which performs correction of an image surface position associated with the variable power and focusing. The second lens group G2 includes at least two negative lenses. A plastic aspherical lens is disposed nearest to an image side in the second lens group G2. When a focal length of the plastic aspherical lens is defined as f2a and a focal length of the second lens group G2 is defined as f2, they satisfy a conditional expression (1). The expression (1) is 2.4&lt;¾f2a/f2¾&lt;10.0. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a zoom lens and an imaging apparatus, and more particularly to a zoom lens that can be suitably used for a video camera, an electronic still camera, a surveillance camera, and the like, and an imaging apparatus including the zoom lens.

  Conventionally, many zoom lenses of the 4-group type and 5-group type have been proposed as zoom lenses used for consumer video cameras, surveillance video cameras, and the like. A zoom lens using an aspherical lens has been proposed as a response to these demands for miniaturization and high performance. In the 4-group type and 5-group type zoom lenses in the above field, there are very many examples in which an aspherical lens is used as the positive lens of the third lens group.

As another example, as described in Patent Documents 1 to 3, there is an example in which an aspherical lens is used for the second lens group which is a variable power group in a four-group type zoom lens. Patent Document 1 describes an example in which the second negative lens from the object side is an aspherical lens in the second lens group having three lenses composed of negative, negative, and positive in order from the object side. Patent Document 2 discloses an example in which the second negative lens from the object side and the positive lens on the image side thereof are aspherical lenses in a second lens group having three elements, negative, negative, and positive, sequentially from the object side. Is described. Patent Document 3 describes an example in which a negative lens closest to the object side is an aspherical lens in the second lens group having four elements including negative, negative, positive, and negative in order from the object side.
JP-A-9-127417 JP 2006-171615 A JP 2006-113387 A

  Incidentally, in recent years, in the zoom lens in the above field, in addition to the demand for downsizing and high performance, the demand for cost reduction is also increasing. One way to reduce costs is to replace an aspheric lens made of a glass mold lens (a lens made of a glass material and formed by molding) with a plastic lens (a lens made of a plastic material). It is done.

  The zoom lenses described in Patent Documents 1 and 2 are a few examples in which the material of the aspherical lens of the second lens group is plastic, and the aspherical lens of the variable power group is a plastic lens.

  However, in the lens types described in Patent Documents 1 to 3, in order to shorten the total lens length, the second lens group often has a strong negative power. The power of individual negative lenses tends to increase. For this reason, when these negative lens materials are made of plastic, there is a problem that a change in characteristics accompanying a change in temperature becomes large.

  For example, the second lens group described in Patent Document 3 uses an aspherical lens having a negative power closest to the object side. However, since this lens has a strong power, it is difficult to configure this lens with plastic.

  In addition, since plastics generally have a low refractive index, there is a problem that the curvature of the lens increases, and the performance deterioration due to lens manufacturing errors and assembly errors becomes very large. If the power of the second lens group is weakened and the power of each negative lens of the second lens group is weakened, it can be made of plastic. The amount of movement at the time of doubling increases, resulting in a problem that the entire lens length becomes longer. In addition, the front lens diameter (the diameter of the lens closest to the object in the entire system) is increased, which is against the size reduction.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a zoom lens that has a small size and good optical performance and is low in cost, and an imaging device including the zoom lens. To do.

The zoom lens of the present invention has, in order from the object side, a first lens group having a positive refractive power and fixed at the time of zooming, a negative refractive power, and moving along the optical axis. A second lens group that performs zooming, a stop, a third lens group that has positive refractive power and is fixed at the time of zooming, and has a positive refractive power, and the position of the image plane associated with zooming A fourth lens group that performs correction and focusing, the second lens group includes at least two negative lenses, and at least one surface is aspherical on the most image side of the second lens group, and is made of a plastic material. An aspheric lens is provided, and when the focal length of the plastic aspheric lens is f2a and the focal length of the second lens group is f2, the following conditional expression (1) is satisfied.
2.4 <| f2a / f2 | <10.0 (1)
Note that f2a is in the paraxial region.

  In the present invention, each “lens group” includes not only a plurality of lenses but also a lens group.

  In the zoom lens of the present invention, by configuring the second lens group to include at least two negative lenses, the negative power necessary for the second lens group is ensured and the entire lens system is reduced in size. By arranging the plastic aspherical lens on the most image side of the second lens group, both good optical performance and cost reduction are achieved. The power of the plastic lens is defined by conditional expression (1), so that performance deterioration due to temperature change and manufacturing / assembly errors is suppressed, and a sufficient aberration correction effect is obtained.

In the zoom lens of the present invention, it is preferable that the following conditional expression (2) is satisfied, where M2 is the amount of movement of the second lens unit in zooming from the wide-angle end to the telephoto end.
2.5 <| M2 / f2 | <3.8 (2)

In the zoom lens of the present invention, it is preferable that the following conditional expression (3) is satisfied, where fw is the focal length of the entire system at the wide angle end and IH is the maximum image height.
1.4 <fw / IH <2.1 (3)

In the zoom lens of the present invention, it is preferable that the following conditional expression (4) is satisfied, where f1 is the focal length of the first lens group.
4.3 <| f1 / f2 | <5.5 (4)

In the zoom lens according to the present invention, the second lens group may include one positive lens. In this case, when the Abbe number of the positive lens is ν2p, the following conditional expression (5 ) Is preferably satisfied.
ν2p <25 (5)

In the zoom lens of the present invention, when the second lens group includes one positive lens, it is preferable that the following conditional expression (6) is satisfied when the refractive index of the positive lens is N2p.
N2p> 1.83 (6)

  In the zoom lens of the present invention, the second lens group may be composed of four lenses in order from the object side: a negative lens, a negative lens, a positive lens, and a plastic aspheric lens. .

In the zoom lens of the present invention, the first lens group may be composed of one negative lens and three or less positive lenses. At that time, when the average Abbe number of the positive lenses constituting the first lens group is ν1p and the Abbe number of the negative lenses constituting the first lens group is ν1n, the following conditional expression (7) is satisfied. preferable.
33 <ν1p−ν1n <50 (7)

In the zoom lens of the present invention, in the most image-side plastic aspheric lens of the second lens group, the distance in the optical axis direction between the effective diameter end on the object-side surface and the object-side vertex on the optical axis, and the image The larger one of the distances in the optical axis direction between the effective diameter end on the side surface and the image side vertex on the optical axis is defined as dZ, and the perpendicular from the effective diameter end to the optical axis on the surface providing dZ It is preferable that the following conditional expression (8) is satisfied, where d is Y.
0.01 <dZ / dY <0.20 (8)

  The values of the conditional expressions are those at the reference wavelength of the zoom lens. For example, when the reference wavelength of the zoom lens is the d-line (wavelength 587.6 nm), the refractive index and Abbe described in the above conditional expressions are used. The numbers are on the d line.

  An image pickup apparatus according to the present invention includes the zoom lens according to the present invention described above.

  According to the present invention, the first lens group and the third lens group are fixed groups, and the second lens group is moved along the optical axis to perform zooming, thereby correcting and focusing the image plane position. In a zoom lens that uses the movement of the fourth lens group, the configuration of the second lens group is suitably set so as to satisfy the conditional expression (1), so that it can be configured compactly and maintains high optical performance. In addition, it is possible to provide a zoom lens in which cost reduction is achieved and an imaging apparatus including the zoom lens.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a cross-sectional view showing a configuration of a zoom lens according to an embodiment of the present invention, and corresponds to a zoom lens of Example 1 described later.

  The zoom lens according to the embodiment of the present invention has, in order from the object side along the optical axis Z, a positive refractive power, a first lens group G1 fixed at the time of zooming, and a negative refractive power. A second lens group G2 that performs zooming by moving along the optical axis Z, an aperture stop St, and a third lens group G3 that has positive refractive power and is fixed during zooming. And a fourth lens group G4 that has a positive refractive power and corrects and focuses an image plane position accompanying zooming.

  Note that the aperture stop St shown in FIG. 1 does not necessarily indicate the size or shape, but indicates the position on the optical axis Z. In FIG. 1, the left side is the object side, and the right side is the image side. In FIG. 1, the upper lens arrangement is shown when focusing on infinity at the wide-angle end, the lower lens arrangement is shown when focusing on infinity at the telephoto end, and each lens group for zooming from the wide-angle end to the telephoto end. The general movement trajectory is indicated by arrows.

  In FIG. 1, the image plane is shown as Sim. For example, when this zoom lens is applied to an image pickup apparatus, the zoom lens is arranged so that the image pickup surface of the image pickup element is positioned on the image plane Sim.

  When applying a zoom lens to an imaging device, depending on the configuration of the camera side on which the lens is mounted, a cover glass, prism, infrared cut filter, low-pass filter, etc. are provided between the lens closest to the image side and the imaging surface. Various filters and the like are preferably arranged, and FIG. 1 shows an example in which a parallel plate-shaped optical member PP assuming these is arranged between the lens group closest to the image side and the image plane Sim.

  In this zoom lens, at the time of zooming from the wide-angle end to the telephoto end, the first lens group G1 and the third lens group G3 are fixed on the optical axis, and the second lens group G2 is imaged along the optical axis. The zoom lens is configured to perform zooming by moving to the side, and to correct and focus the image plane position accompanying the zooming by moving the fourth lens group G4 along the optical axis. That is, the second lens group G2 functions as a variator group, and the fourth lens group G4 functions as a compensator group and a focus group.

  The zoom lens according to this embodiment has a characteristic configuration of the second lens group G2. The second lens group G2 includes at least two negative lenses, and a plastic aspherical lens made of a plastic material and having at least one aspherical surface is disposed on the most image side of the second lens group G2.

  For example, the second lens group G2 in the example illustrated in FIG. 1 includes a meniscus negative lens L21, a cemented lens formed by bonding a biconcave negative lens L22, and a biconvex positive lens L23 in order from the object side. And four lenses L24. The negative lens L21 and the negative lens L22 in the example shown in FIG. 1 are glass spherical lenses. The lens L24 is a plastic aspheric lens having both aspheric surfaces.

  By providing an aspherical lens on the most image side of the second lens group G2, it is possible to suppress aberration fluctuations, image plane fluctuations, and distortion during zooming. Further, the lens L24 is made of plastic, so that the cost and weight can be reduced. Since the plastic lens has a higher degree of freedom in shape than the glass lens, it becomes very useful by setting the position and power to be arranged well.

  However, as described above in the problem section, when using a plastic lens, it should be noted that the characteristic change due to temperature change is large. Therefore, in this embodiment, the lens is configured so as to satisfy the following conditional expression (1), and is set so that the power of the lens L24 is weak.

2.4 <| f2a / f2 | <10.0 (1)
Here, f2a is the focal length of the plastic aspherical lens closest to the image side of the second lens group G2, and f2 is the focal length of the second lens group G2.

  Conditional expression (1) defines the ratio between the focal length of the plastic aspherical lens closest to the image side of the second lens group G2 and the focal length of the second lens group G2. The ratio of power to the two lens group G2 is defined. Conditional expression (1) can be said to be a condition for forming the most aspherical lens on the image side of the second lens group G2 with plastic. If the power of the plastic aspherical lens closest to the image side in the second lens group G2 is increased as the lower limit of conditional expression (1) is exceeded, the change in characteristics due to a temperature change will increase. In addition, since the refractive index of plastic is generally low, if the power of the plastic aspherical lens closest to the image side in the second lens group G2 is increased as it falls below the lower limit of the conditional expression (1), the curvature of the lens increases. Therefore, the performance deterioration due to the manufacturing error and assembly error of the lens becomes very large. If the power of the plastic aspherical lens closest to the image side in the second lens group G2 is weakened as the upper limit of conditional expression (1) is exceeded, the curvature of the lens becomes small and aberrations cannot be corrected sufficiently.

  In other words, the lens L24 has little influence on power distribution and serves as an “aberration correction lens” for satisfactorily correcting aberrations. By using the lens L24 as such a lens, it is possible to reduce the influence of temperature change and maintain good optical performance while the lens L24 is made of plastic.

  Another point to note is that since plastic is a low refractive index material, the greater the power it has, the greater the curvature of the lens and the greater the performance degradation associated with manufacturing and assembly errors. is there. In the present embodiment, the lens L24 is treated as an “aberration correction lens” and its power is set to be weak, so that it is not necessary to increase the curvature, and this problem can be solved.

  The negative power necessary for the second lens group G2 can be obtained by configuring the second lens group G2 to include at least two negative lenses. In the example shown in FIG. 1, two negative lenses L21 and L22 are provided, and the negative lens L22 has a biconcave shape. With such a configuration, it is possible to secure a strong negative power necessary for the second lens group G2, to reduce the amount of movement during zooming, and to achieve miniaturization.

  In particular, as in the example shown in FIG. 1, when two negative lenses are arranged on the object side of the second lens group G2, a large amount of negative power is distributed to the object side, so that the second lens group The distance between the principal points of the first lens group G1 and the second lens group G2 can be shortened by bringing the position of the object side principal point closer to the object side. As a result, the height of the off-axis light beam passing through the first lens group G1 can be reduced, and the front lens diameter can be reduced.

  The second lens group G2 of this embodiment will be considered in comparison with the conventional one. The second lens group described in Patent Literatures 1 and 2 has a negative, negative, and positive three-lens configuration, and the second negative lens from the object side is a plastic aspheric lens. This plastic aspherical lens has a biconcave shape and is considered to play a major role not only in aberration correction but also in power distribution. In the configurations described in Patent Documents 1 and 2, if the power of the negative lens is weakened in order to reduce the influence of temperature change or reduce the curvature of the lens, the power of the second lens group is weakened. As a result, the amount of movement during zooming increases, and the system becomes larger.

  On the other hand, the second lens group of the present embodiment shown in FIG. 1 employs a configuration in which one plastic aspherical lens having a weak power is arranged on the image side of a negative, negative, and positive three-lens configuration. The influence of the temperature change is small, it is not necessary to increase the curvature of the plastic aspheric lens, the negative power of the second lens group G2 can be secured, and the system can be miniaturized.

  And in this embodiment, unlike the thing of the patent documents 1 and 2, the 2nd negative lens from an object side can be used as a glass spherical lens. By making this negative lens a glass lens, it becomes easy to give a strong power, and it is possible to reduce the amount of movement at the time of zooming and to reduce the size. In addition, the plastic lens has a low material selectivity, whereas the glass lens has a high material selectivity. Therefore, the glass lens is advantageous in correcting chromatic aberration. In particular, in the case where the second negative lens from the object side of the second lens group forms a cemented lens with an adjacent positive lens, selection of a material for correcting chromatic aberration is important. Further, regarding the production of the cemented lens, the glass lens is more advantageous than the plastic lens.

  The second lens group described in Patent Document 3 has a negative, negative, positive, and negative four-lens configuration, and the number of lenses is the same as that of the present embodiment. However, in Patent Document 3, since the most object side lens having strong power is an aspherical lens, it is difficult to configure this lens with plastic. On the other hand, in the present embodiment, among the lenses included in the second lens group G2, the temperature change is achieved by applying an aspherical surface to the lens having the relatively weak power on the most image surface side to form a plastic lens. Can be reduced, and the cost and weight can be reduced.

  In other words, the lens L24, which is made of an inexpensive and lightweight plastic but is less affected by temperature changes and has a good aberration correction capability, is used in the prior art on the most image side of the second lens group G2. In addition, it is possible to reduce the glass aspherical lens, to reduce the cost, and to obtain a zoom lens having a small size and a well-corrected aberration.

  Note that the configuration of the second lens group G2 is not limited to the example shown in FIG. For example, the most image-side plastic aspheric lens of the second lens group G2 may have a negative power or a positive power in the vicinity of the optical axis. The second lens group G2 includes, in order from the object side, a negative lens, a negative lens, a negative lens, a positive lens, at least one aspherical surface, and a non-plastic plastic made of a plastic material, as will be described later. The spherical lens may have a five-lens configuration, and in this case, the negative power necessary for the second lens group G2 can be further dispersed, so that the power of each negative lens can be suppressed, or the negative power stronger by the second lens group G2. Can be given. In addition, the second lens group G2 may be configured to have a cemented lens obtained by bonding a negative lens and a positive lens. In this case, it is advantageous for correcting chromatic aberration.

  The first lens group G1 may be composed of one negative lens and three or less positive lenses. For example, as shown in FIG. 1, the meniscus shape is formed in order from the object side. A four-lens configuration including a cemented lens obtained by bonding the negative lens L11 and the meniscus positive lens L12, a meniscus positive lens L13, and a meniscus positive lens L14 may be used.

  As the configuration of the third lens group G3, for example, as shown in FIG. 1, in the paraxial region, a meniscus positive lens L31, a meniscus positive lens L32, and a meniscus negative lens L33 are bonded together. A three-lens configuration including lenses may be used.

  The configuration of the fourth lens group G4 may be a two-lens configuration including a biconvex positive lens L41 and a meniscus negative lens L42 in the paraxial region as in the example shown in FIG.

  In addition, as shown in Examples described later, a configuration in which a lens group is further provided on the image side of the fourth lens group G4 is also possible.

  In addition to the above-described configuration, the zoom lens according to the embodiment of the present invention is preferably configured to satisfy the following conditional expression, whereby even better characteristics can be obtained. As a preferred embodiment, any one of the following conditional expressions may be satisfied, or any combination may be satisfied.

When the focal length of the plastic aspheric lens closest to the image side of the second lens group G2 is f2a and the focal length of the second lens group G2 is f2, it is preferable that the following conditional expression (1-1) is satisfied.
2.4 <f2a / f2 <10.0 (1-1)

  Conditional expression (1-1) means that the above-mentioned conditional expression (1) is satisfied, and that the most image-side plastic aspherical lens in the second lens group G2 is a negative lens in the paraxial region. is there. When the conditional expression (1-1) is satisfied, in addition to the effect of satisfying the conditional expression (1), the negative power of the second lens group G2 can be dispersed. It is possible to suppress the lens power from becoming too strong. This makes it easy to suppress aberration fluctuations and image plane fluctuations during zooming while reducing the overall length of the lens system.

When the amount of movement of the second lens group G2 at the time of zooming from the wide-angle end to the telephoto end is M2, and the focal length of the second lens group G2 is f2, it is preferable that the following conditional expression (2) is satisfied.
2.5 <| M2 / f2 | <3.8 (2)

  Conditional expression (2) defines the ratio between the movement amount of the second lens group G2 and the focal length of the second lens group G2. If the lower limit of conditional expression (2) is not reached, it will be difficult to increase the magnification. If the upper limit of conditional expression (2) is exceeded, the amount of movement of the second lens group G2 will increase and the total lens length will increase. Alternatively, the Petzval sum increases in the negative direction, and the field curvature increases, which is not preferable.

Furthermore, it is more preferable to satisfy the following conditional expression (2-1). By satisfying conditional expression (2-1), the effect obtained by satisfying conditional expression (2) can be further enhanced.
2.6 <| M2 / f2 | <3.7 (2-1)

When the focal length of the entire system at the wide angle end is fw and the maximum image height is IH, it is preferable that the following conditional expression (3) is satisfied.
1.4 <fw / IH <2.1 (3)

  Conditional expression (3) defines the ratio between the focal length of the entire system at the wide-angle end and the maximum image height. In the zoom lens configured as in the present embodiment, if an attempt is made to widen the angle so as to fall below the lower limit of the conditional expression (3), the off-axis light beam incident on the first lens group G1 at the intermediate zoom position from the wide angle end. The height of the light beam becomes large, and an increase in the diameter of the first lens group G1 is unavoidable. In addition, the power of the second lens group G2 becomes too strong, and the power of each lens included in the second lens group G2 becomes strong, and aberration fluctuation and image plane fluctuation at the time of zooming become large. If the upper limit of conditional expression (3) is exceeded, the angle of view of the video camera or surveillance video camera will be insufficient.

Furthermore, it is more preferable to satisfy the following conditional expression (3-1). By satisfying conditional expression (3-1), the effect obtained by satisfying conditional expression (3) can be further enhanced.
1.5 <fw / IH <2.0 (3-1)

When the focal length of the first lens group G1 is f1, and the focal length of the second lens group G2 is f2, it is preferable that the following conditional expression (4) is satisfied.
4.3 <| f1 / f2 | <5.5 (4)

  Conditional expression (4) defines the ratio of the focal lengths of the first lens group G1 and the second lens group G2, and is a condition for maintaining good optical performance with a high-magnification and compact configuration. It is. The lower the lower limit of conditional expression (4), the larger the focal length of the second lens group G2, and the smaller the focal length of the first lens group G1, the greater the amount of movement of the second lens group G2 with zooming. It becomes difficult to reduce the total lens length and front lens diameter. In addition, the amount of movement of the fourth lens group G4 on the telephoto side increases, and the variation in aberration during zooming increases. On the contrary, if the upper limit of conditional expression (4) is exceeded, it becomes difficult to satisfactorily correct various aberrations such as distortion.

Furthermore, it is more preferable to satisfy the following conditional expression (4-1). By satisfying conditional expression (4-1), the effect obtained by satisfying conditional expression (4) can be further enhanced.
4.4 <| f1 / f2 | <5.4 (4-1)

When the second lens group G2 includes one positive lens, it is preferable that the following conditional expression (5) is satisfied when the Abbe number of the positive lens is ν2p.
ν2p <25 (5)

  Conditional expression (5) defines the Abbe number of the positive lens included in the second lens group G2, and by satisfying this, there is only one positive lens included in the second lens group G2. However, it becomes possible to correct chromatic aberration satisfactorily. If the Abbe number increases as the upper limit of conditional expression (5) is exceeded, the effect of chromatic aberration correction in the second lens group G2 will be reduced, and it will be difficult to achieve high zooming and high performance.

Furthermore, it is more preferable to satisfy the following conditional expression (5-1). By satisfying conditional expression (5-1), the effect obtained by satisfying conditional expression (5) can be further enhanced.
ν2p <21 (5-1)

When the second lens group G2 includes one positive lens, it is preferable that the following conditional expression (6) is satisfied when the refractive index of the positive lens is N2p.
N2p> 1.83 (6)

  Conditional expression (6) defines the refractive index of the positive lens included in the second lens group G2. If the lower limit of conditional expression (6) is not reached, it will be difficult to suppress fluctuations in coma upon zooming.

Furthermore, it is more preferable to satisfy the following conditional expression (6-1). By satisfying conditional expression (6-1), the effect obtained by satisfying conditional expression (6) can be further enhanced.
N2p> 1.90 (6-1)

When the first lens group G1 is composed of one negative lens and three or less positive lenses, the average of the Abbe numbers of the positive lenses constituting the first lens group G1 is ν1p, and the first lens group When the Abbe number of the negative lens constituting G1 is ν1n, it is preferable that the following conditional expression (7) is satisfied.
33 <ν1p−ν1n <50 (7)

  Conditional expression (7) defines the relationship between the average Abbe number of the positive lenses constituting the first lens group G1 and the Abbe number of the negative lenses constituting the first lens group G1. Below the lower limit of conditional expression (7), the refractive index of the positive lens increases, which is advantageous for downsizing the first lens group, but it is difficult to correct chromatic aberration, particularly chromatic aberration on the telephoto side. . Using a material with small dispersion for the positive lens so as to exceed the upper limit of conditional expression (7) is advantageous for correcting chromatic aberration, but the refractive index of the positive lens is lowered and the curvature of the positive lens is increased. In order to secure the rim of these lenses, the first lens group G1 is increased in size.

Furthermore, it is more preferable to satisfy the following conditional expression (7-1). By satisfying conditional expression (7-1), the effect obtained by satisfying conditional expression (7) can be further enhanced.
35 <ν1p−ν1n <48 (7-1)

In the most image side plastic aspherical lens of the second lens group G2, the distance in the optical axis direction between the effective diameter end on the object side surface and the object side vertex on the optical axis, and the effective diameter on the image side surface The larger one of the intervals in the optical axis direction between the end and the image-side vertex on the optical axis is dZ, and the length of the perpendicular from the end of the effective diameter to the optical axis on the surface on which dZ is given is dY It is preferable that the following conditional expression (8) is satisfied.
0.01 <dZ / dY <0.20 (8)

  FIG. 10 shows an example of the dZ and dY. FIG. 10 shows a sectional view of a lens L24, which is the most image-side plastic aspherical lens in the second lens group G2, and an example of a light beam Mg passing through the effective diameter end below the optical axis Z of the lens L24. Yes. The light beam Mg is a light beam that determines the effective diameter end. In FIG. 10, the left side of the figure is the object side, and the right side of the figure is the image side, and in order to avoid complication of the figure, a part of the light beam Mg and other lenses are not shown.

In the example shown in FIG. 10, the surface on the object side is a side on which give dZ, the interval between the optical axis between the object side apex P ₀ on the effective diameter edge Ped and the optical axis at the object-side surface dZ, the length of the perpendicular line dropped from the effective diameter end Ped to the optical axis Z is dY.

  Conditional expression (8) defines the ratio of the sag amount at the effective diameter end of the plastic aspherical lens closest to the image side of the second lens group G2 and the distance from the effective diameter end to the optical axis. If the lower limit of conditional expression (8) is not reached, the curvature of this lens becomes small, and aberrations cannot be corrected sufficiently. If the upper limit of conditional expression (8) is exceeded, the total length of the second lens group G2 will be long. In addition, an allowable amount of error such as misalignment or tilting of the lenses constituting the second lens group G2 during assembly is reduced.

Furthermore, it is more preferable to satisfy the following conditional expression (8-1). By satisfying conditional expression (8-1), the effect obtained by satisfying conditional expression (8) can be further enhanced.
0.02 <dZ / dY <0.18 (8-1)

  In addition, when this zoom lens is used in harsh environments such as outdoors, the lens placed closest to the object is resistant to surface deterioration due to wind and rain, temperature changes due to direct sunlight, and oils and detergents. It is preferable to use a material resistant to chemicals, that is, a material having high water resistance, weather resistance, acid resistance, chemical resistance, and the like, and further, a material that is hard and difficult to break. From the above, as the material arranged closest to the object side, specifically, glass is preferably used, or transparent ceramics may be used.

  When the zoom lens is used in a harsh environment, a protective multilayer coating is preferably applied. In addition to the protective coat, an antireflection coating film for reducing ghost light during use may be applied.

  In the example shown in FIG. 1, an example in which the optical member PP is arranged between the lens system and the image plane Sim is shown, but instead of arranging a low-pass filter, various filters that cut a specific wavelength range, or the like, These various filters may be disposed between the lenses, or a coating having the same action as the various filters may be applied to the lens surface of any lens.

  As described above, according to the zoom lens of the present embodiment, according to the required specifications and the like, by appropriately adopting the preferred configuration described above, it can be configured compactly without significantly increasing the number of lenses, It is possible to achieve both good aberration correction and cost reduction.

  Next, numerical examples of the zoom lens according to the present invention will be described. A lens cross-sectional view of the zoom lens of Example 1 is shown in FIG. FIGS. 2 to 9 show sectional views of the zoom lenses of Examples 2 to 9, respectively. 2 to 9 are the same as those shown in FIG.

  Table 1 shows basic lens data of the zoom lens according to Example 1, Table 2 shows data relating to zooming (magnification), and Table 3 shows aspherical data. Similarly, Tables 4 to 27 show basic lens data, zoom-related data, and aspherical data of the zoom lenses according to Examples 2 to 9, respectively. In the following, the meaning of the symbols in the table will be described using Example 1 as an example, but the same applies to Examples 2 to 9.

  In the basic lens data of Table 1, Si indicates the i-th (i = 1, 2, 3,...) Surface number that increases sequentially toward the image side with the most object-side component surface being first. Indicates the radius of curvature of the i-th surface, and Di indicates the surface interval on the optical axis Z between the i-th surface and the i + 1-th surface. The bottom column of the surface interval indicates the surface interval between the final surface in the table and the image surface Sim. In the basic lens data, Ndj is the d-line (wavelength: 587.6 nm) of the j-th (j = 1, 2, 3,...) Optical element that sequentially increases toward the image side with the most object-side lens as the first lens. ), And νdj represents the Abbe number of the j-th optical element with respect to the d-line. The basic lens data includes the aperture stop St and the optical member PP. In the column of the radius of curvature of the surface corresponding to the aperture stop St, (aperture stop) is described. The sign of the radius of curvature of the basic lens data is positive when convex on the object side and negative when convex on the image side.

  In the basic lens data in Table 1, the interval changes for zooming, the interval between the first lens group G1 and the second lens group G2, the interval between the second lens group G2 and the aperture stop St, the third lens group G3. D7 (variable), D14 (variable), D20 (variable), D24 (variable) in the columns of the distance between the fourth lens group G4 and the surface distance corresponding to the distance between the fourth lens group G4 and the optical member PP, respectively. It is described.

  In addition, although the code | symbol of the surface interval from which a space | interval changes in order to perform magnification change is as above-mentioned about Example 1, in the below-mentioned Example, the code | cord | chord according to each structure will be taken.

  The zoom-related data in Table 2 includes the focal length f of the entire system at the wide-angle end and the telephoto end, the F number Fno. , The total field angle 2ω, and the values of the surface spacings D7, D14, D20, and D24 that change with zooming. The unit of the total angle of view 2ω is degrees.

  As the Ri and Di units in Table 1 and the units f, D7, D14, D20, and D24 in Table 2, "mm" can be used. Since the performance can be obtained, the unit is not limited to “mm”, and other appropriate units can be used.

In the basic lens data in Table 1, the surface number of the aspheric surface is marked with *, and the paraxial radius of curvature is shown as the radius of curvature of the aspheric surface. The aspherical data in Table 3 shows the sign of a lens that is an aspherical lens, the surface number of the aspherical surface, and the aspherical coefficients related to these aspherical surfaces. The aspheric coefficient is a value of each coefficient KA, RA _m (m = 3, 4, 5,... 10) in the aspheric expression expressed by the following expression (A).

Zd = C · h ² / {1+ (1−KA · C ² · h ² ) ^1/2 } + ΣRA _m · h ^m (A)
However,
Zd: Depth of aspheric surface (length of perpendicular drawn from a point on the aspherical surface of height h to a plane perpendicular to the optical axis where the aspherical vertex contacts)
h: Height (distance from the optical axis to the lens surface)
C: Reciprocal number of paraxial radius of curvature KA, RA _m : aspheric coefficient (m = 3, 4, 5,... 10)
In addition, when mm is used as the unit of Ri and Di in Table 1, the unit of Zd and h is also mm.

  The power in the paraxial region of the most image-side plastic aspheric lens in the second lens group G2 is positive in Examples 1 and 9, and negative in Examples 2-8.

  The zoom lens according to the third embodiment is greatly different from that according to the first embodiment in that the first lens group G1 includes a three-lens configuration including a negative lens L11, a positive lens L12, and a positive lens L13.

  The zoom lens of Example 5 is significantly different from that of Example 1 in that the first lens group G1 has a three-lens configuration including a negative lens L11, a positive lens L12, and a positive lens L13.

  In the zoom lens of Example 6, the first lens group G1 has a three-lens configuration including a negative lens L11, a positive lens L12, and a positive lens L13. The third lens group G3 is a single lens, and the fourth lens group G4. Is significantly different from that of the first embodiment in that it is a single lens configuration of the positive lens L41.

  In the zoom lens of Example 8, the second lens group G2 includes a meniscus negative lens L21, a biconcave negative lens L22, a biconcave negative lens L23, a biconvex positive lens L24, and a paraxial region. The lens L25, which is a biconcave plastic aspherical lens, has a five-lens configuration consisting of all single lenses, and further includes a fifth lens group G5 consisting of one negative lens L51. Different. The fifth lens group G5 in Embodiment 8 is fixed at the time of zooming, and the correction and focusing of the image plane position accompanying zooming is performed by the fourth lens group G4.

  The zoom lens of Example 9 is significantly different from that of Example 1 in that the second lens group G2 is entirely a single lens and further includes a fifth lens group G5 including one positive lens L51. The fifth lens group G5 in Example 9 is fixed at the time of zooming, and the correction and focusing of the image plane position accompanying zooming is performed by the fourth lens group G4.

  Table 28 shows values corresponding to the conditional expressions (1) to (8) in Examples 1 to 9. As can be seen from Table 28, all of Examples 1 to 9 satisfy the conditional expressions (1) to (8).

  FIGS. 11A to 11H are graphs showing spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration at the wide-angle end and the telephoto end of the zoom lens according to the first exemplary embodiment. Each aberration diagram shows the aberration with the d-line (wavelength 587.6 nm) as the reference wavelength, while the spherical aberration diagram and the magnification chromatic aberration diagram also show the aberrations for the wavelength 460.0 nm and the wavelength 615.0 nm. Fno. Of spherical aberration diagram. Means F number, and ω in other aberration diagrams means half angle of view.

  Similarly, FIGS. 12 (A) to 12 (H), FIGS. 13 (A) to 13 (H), FIGS. 14 (A) to 14 (H), and FIGS. 15 (A) to 15 (H). 16 (A) to FIG. 16 (H), FIG. 17 (A) to FIG. 17 (H), FIG. 18 (A) to FIG. 18 (H), FIG. 19 (A) to FIG. FIG. 9 shows aberration diagrams of spherical aberration, astigmatism, distortion (distortion aberration), and lateral chromatic aberration at the wide-angle end and the telephoto end of the zoom lenses of Examples 2 to 9.

  From the above data, the zoom lenses of Examples 1 to 9 have a magnification of about 10 to 20 times, and the F number at the wide-angle end is as small as about 1.9, and each aberration is good while achieving downsizing. It can be seen that both the wide-angle end and the telephoto end have high optical performance in the visible range. These zoom lenses can be suitably used for imaging devices such as surveillance cameras, video cameras, and electronic still cameras.

  FIG. 20 is a configuration diagram of a video camera 10 configured using the zoom lens 1 according to the embodiment of the present invention as an example of the imaging device of the embodiment of the present invention. In FIG. 20, the positive first lens group G1, the negative second lens group G2, the aperture stop St, the positive third lens group G3, and the positive fourth lens group G4 included in the zoom lens 1 are schematically illustrated. Show.

  The video camera 10 includes a zoom lens 1, a filter 2 having functions such as a low-pass filter and an infrared cut filter disposed on the image side of the zoom lens 1, an image pickup device 4 disposed on the image side of the filter 2, and a signal. And a processing circuit 5. The image sensor 4 converts an optical image formed by the zoom lens 1 into an electric signal. For example, the image sensor 4 may be a CCD (Charge Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor), or the like. it can. The image sensor 4 is arranged such that its image plane coincides with the image plane of the zoom lens 1.

  An image picked up by the zoom lens 1 is formed on the image pickup surface of the image pickup device 4, an output signal from the image pickup device 4 relating to the image is subjected to arithmetic processing by the signal processing circuit 5, and the image is displayed on the display device 6. The

  FIG. 20 shows a so-called single-plate type imaging apparatus using one imaging element 4. However, as an imaging apparatus according to the present invention, R ( A so-called three-plate system may be used in which color separation prisms for each color such as red), G (green), and B (blue) are inserted and three image pickup devices corresponding to the respective colors are used.

  Since the zoom lens according to the embodiment of the present invention has the above-described advantages, the imaging apparatus of the present embodiment can be configured in a small size and at a low cost, and can obtain a high-quality image.

  The present invention has been described with reference to the embodiments and examples. However, the present invention is not limited to the above embodiments and examples, and various modifications can be made. For example, the values of the radius of curvature, the surface spacing, the refractive index, the Abbe number, etc. of each lens component are not limited to the values shown in the above numerical examples, but can take other values.

Sectional drawing which shows the lens structure of the zoom lens concerning Example 1 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 2 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 3 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 4 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 5 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 6 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 7 of this invention. Sectional drawing which shows the lens structure of the zoom lens concerning Example 8 of this invention. Sectional view showing the lens configuration of a zoom lens according to Example 9 of the present invention. Cross section of plastic aspherical lens and diagram showing light rays passing through effective diameter end Each aberration diagram of the zoom lens according to Example 1 of the present invention Each aberration diagram of the zoom lens according to Example 2 of the present invention Each aberration diagram of the zoom lens according to Example 3 of the present invention Each aberration diagram of the zoom lens according to Example 4 of the present invention Each aberration diagram of the zoom lens according to Example 5 of the present invention Each aberration diagram of the zoom lens according to Example 6 of the present invention Each aberration diagram of the zoom lens according to Example 7 of the present invention Each aberration diagram of the zoom lens according to Example 8 of the present invention Each aberration diagram of the zoom lens according to Example 9 of the present invention 1 is a schematic configuration diagram of an imaging apparatus according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Zoom lens 2 Filter 4 Image pick-up element 5 Signal processing circuit 6 Display apparatus 10 Video camera G1 1st lens group G2 2nd lens group G3 3rd lens group G4 4th lens group G5 5th lens group PP Optical member St Aperture stop Z optical axis

Claims (11)

In order from the object side, a first lens group having positive refractive power and fixed at the time of zooming, and a second lens having negative refractive power and zooming by moving along the optical axis A third lens group having a positive refracting power and fixed at the time of zooming, a first lens having a positive refracting power, and correcting and focusing the image plane position accompanying the zooming. With 4 lens groups,
The second lens group includes at least two negative lenses, and on the most image side of the second lens group, a plastic aspheric lens having at least one aspheric surface and made of a plastic material is disposed,
A zoom lens satisfying the following conditional expression (1) when the focal length of the plastic aspheric lens is f2a and the focal length of the second lens group is f2.
2.4 <| f2a / f2 | <10.0 (1) 2. The zoom lens according to claim 1, wherein the following conditional expression (2) is satisfied, where M <b> 2 is an amount of movement of the second lens unit in zooming from the wide-angle end to the telephoto end.
2.5 <| M2 / f2 | <3.8 (2) 3. The zoom lens according to claim 1, wherein the following conditional expression (3) is satisfied when a focal length of the entire system at the wide angle end is fw and a maximum image height is IH.
1.4 <fw / IH <2.1 (3) 4. The zoom lens according to claim 1, wherein the following conditional expression (4) is satisfied, where f1 is a focal length of the first lens group. 5.
4.3 <| f1 / f2 | <5.5 (4) The said 2nd lens group contains one positive lens, and when the Abbe number of this positive lens is set to (nu) 2p, the following conditional expression (5) is satisfy | filled, The any one of Claim 1 to 4 characterized by the above-mentioned. The described zoom lens.
ν2p <25 (5) The said 2nd lens group contains one positive lens, and when the refractive index of this positive lens is set to N2p, the following conditional expression (6) is satisfy | filled, The any one of Claim 1 to 5 characterized by the above-mentioned. The described zoom lens.
N2p> 1.83 (6)   The said 2nd lens group is comprised from 4 elements of a negative lens, a negative lens, a positive lens, and the said plastic aspherical lens in order from an object side. The zoom lens according to item 1.   The zoom lens according to claim 1, wherein the first lens group includes one negative lens and three or less positive lenses. When the average Abbe number of the positive lens constituting the first lens group is ν1p and the Abbe number of the negative lens constituting the first lens group is ν1n, the following conditional expression (7) is satisfied. The zoom lens according to claim 8.
33 <ν1p−ν1n <50 (7) In the plastic aspheric lens of the second lens group, the distance in the optical axis direction between the effective diameter end on the object side surface and the object side vertex on the optical axis, and the effective diameter end on the image side surface and the optical axis. When the larger one of the distances from the image-side vertex to the optical axis direction is dZ, and the length of the perpendicular line from the effective diameter end to the optical axis on the surface on the side where dZ is given is dY, the following condition The zoom lens according to claim 1, wherein Expression (8) is satisfied.
0.01 <dZ / dY <0.20 (8)   An imaging apparatus comprising the zoom lens according to claim 1.

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